This invention generally relates to process control and more specifically to a method and apparatus for accurately and automatically maintaining the cut length thickness of a continuously extruded elastomer that may be applied as tread stock in the manufacture of vehicle tires.
Presently, the elastomeric stock used for tire treads is extruded in a continuous sheet form of the desired width, cooled, and cut to the required length for a particular tire circumference. The thickness of tread stock has heretofore been controlled by an extruder operator monitoring the weight of the extruded material since its weight is directly related to the material thickness.
Monitoring of the extrusion is accomplished manually by an operator comparing the weight of a running length of continuous extrusion as it passes over a running-weigh-scale (hereinafter referred to as the RWS) to the weight of a cut length of the material taken at a check weight scale (hereinafter referred to as the CWS) after the extrusion is cooled and skived. The tread stock specification is established at the CWS with the RWS providing a means for the operator to make a first approximation inasmuch as the RWS and CWS are at separate locations in the process. If, for example, the operator has determined that the weight and thus the thickness of cut tread stock is too high as taken at the CWS, he may vary the speed of a take-away belt positioned at the exit orifice of the extruder. By increasing the belt speed, the extrusion is stretched and therefore a thinner tread stock will result. When the CWS value of tread stock is within specification, the operator again checks the weight value at the RWS that produced the correct tread stock thickness and thereafter attempts to maintain the RWS at this value. Alternately, if the CWS value is too low, indicative of a thinner tread stock, the take-away belt may be slowed down with the result being a thicker extrusion.
Thus, it must be appreciated that the present system of maintaining cut length extrusion thickness is primarily an operator oriented operation and leaves much to be desired. For example, the operator is limited in the number of checks that may be made during the time he is "zeroing-in" on the correct RWS weight a length of extrusion will have passed that may have to be scraped because it is out of specification as determined at the CWS. Further, the accuracy of the adjustments are at most a hit-and-miss proposition being dependent on the operator doing the checks. In addition, changes in the extruded material are continually taking place due to environmental conditions and it is difficult to monitor the material often enough to maintain continual quality of cut tread lengths.
It is therefore an object of this invention to provide an automated control system that is fast and accurate and which is capable of maintaining tread stock thickness within a closer tolerance than heretofore possible.
Another object of the invention is to provide a method and apparatus capable of detecting system malfunctions at an early stage such as to eliminate long lengths of extruded scrap material.
A further object is to ultimately produce tire tread extrusions that exhibit uniformity in thickness and therefore offer increased quality than heretofore possible.